These races will determine control of the Senate

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Republicans must pick up six seats to win control of the Senate

Six Democratic races and two Republican races are key to the balance of power

In the battle for the Senate, nine races will play a role in determining the balance of power. Democrats currently hold six of the seats while Republicans hold the other two, meaning that the Democrats must defend their territory -- a difficult task in a midterm election with a Democratic president who has low approval ratings.

Here's a short synopsis of the key races that Democrats are working to keep in their hands:

Alaska: Sen. Mark Begich is another freshman senator who is facing a difficult road to reelection. He won his last race by fewer than 4,000 votes in the red state. He is distancing himself from President Barack Obama and touting his pro-energy credentials. He could face one of three Republicans. Their primary is August 19.

Arkansas: This is one of the most Republican states in the country. President Barack Obama received only 37% of the vote in 2012 and Sen. Mark Pryor is locked in a race with Rep. Tom Cotton, a Republican backed by both conservative and establishment elements of the Republican Party.

Louisiana: Since her first Senate race in 1996, Sen. Mary Landrieu has had a tough road to election. Now running for her fourth term, Republicans are still trying to unseat the centrist Democrat. This year she will face off against one of several Republican challengers, including Rep. Bill Cassidy, who is considered her biggest threat.

Louisiana runs its elections a little differently. November 4, Election Day for the rest of the country, is considered Louisiana's primary. And if no candidate receives 50% of the vote, a runoff takes place December 6.

Michigan: Long-time Sen. Carl Levin is retiring, giving Republicans their first chance to take a seat in Michigan since 1994. Republican Terry Lynn Land is staging a competitive race against Democratic Rep. Gary Peters.

Montana: Cook Political Report categorizes this race as "lean Republican." Sen. John Walsh is running as an incumbent, appointed to the seat after Max Baucus left to become ambassador to China, but that status is thin in the conservative state as he's only been in office since February. Rep. Steve Daines is the likely Republican candidate.

North Carolina: Sen. Kay Hagan is running for her second term. It's still unclear who she will face in the November election as Republicans are battling it out for that party's nomination. But her likely challenger is state House Speaker Thom Tillis. While she will have to run against her Republican opponent, she also will have to run against her public perception as she faces approval ratings of only 35%, according to an April 8 High Point University/University of North Carolina-Chapel Hill poll.

Iowa: Sen. Tom Harkin's retirement opens the seat up to a possible Republican win. Rep. Bruce Braley will be the Democratic candidate and the Republican field is crowded. But if no GOP candidate obtains 35% support in the primary, then the candidate is chosen by a conservative state convention.

Republicans are also playing in Colorado and New Hampshire, which means they have multiple paths to retaking the Senate.

But Republicans are defending two key seats that could make it more difficult for them to pick up the majority.

Kentucky: Republican leader Mitch McConnell is in the race of his life. He is being attacked by the tea party wing of the party with a challenger, Matt Bevin, who he's likely to beat in the May 20 primary. Then Democrats have a legitimate chance in the red state with their candidate, Alison Lundergan Grimes.

Georgia: The current Republican senator, Saxby Chambliss, is retiring, leaving his seat vulnerable. Michelle Nunn comes from a family steeped in Georgia politics and is proving to be a viable Democratic candidate in this red state. Over on the GOP side, the candidates are beating each other up in a contentious primary and pushing each other further to the right, which could leave the primary winner bruised and too extreme for a general election electorate. Additionally, primary candidates must win more than 50% of the vote to avoid a runoff. With the crowded field, a runoff is possible, adding another challenging element to Republican efforts to keep this seat.